1. Technical Field
Embodiments of the present invention relate to methods and apparatus for enhancing transmission and reception of signals between groups of communication units and, more particularly, to techniques for synchronously combining coordinated transmissions from plural communication units of a group at receiving communication units within a distant group to extend range performance while maintaining the communication protocol employed for the communication.
2. Discussion of the Related Art
One factor affecting range performance in wireless communication systems is the power with which signals are transmitted. Generally, the strength of a received signal is proportional to the transmit power and inversely proportional to the nth power of the range between the transmitter and receiver. Depending on the underlying terrain, the value of n could generally range from 2 to 6. At a given transmit power, as the range between a transmitting device and a receiving device increases, the signal strength at the receiving device becomes increasingly attenuated, eventually prohibiting reception. Range performance improves with increasing transmit power; conversely, lower transmit power reduces the maximum range at which transmitted signals can be detected.
Unfortunately, there are a number of circumstances in which transmit power is limited by equipment capabilities, operational requirements or both. For example, with mobile communication devices which rely on battery power supplies, the maximum transmit power may be limited by design to achieve a tradeoff between operating range and battery power consumption. Other equipment cost or performance considerations may dictate transmit power capabilities that limit operational range or limit system performance under harsh operating conditions.
In certain circumstances, a need may exist to minimize RF emissions. In military contexts, particularly in battlefield situations, minimizing transmit power reduces the likelihood of signal detection by hostile parties, thereby preventing the transmitter's position from being compromised. For example, in a situation where a small team of radio-equipped personnel is located within a hostile region and must communicate with a distant device, for any single radio to communicate with the distant device, considerable transmit power would be required, resulting in a significant opportunity for detection by hostile forces. Moreover, the transmit power level required to communicate over significant distances would likely drain the radio's battery more rapidly than desired. In other contexts, low transmit power levels may minimize interference with other devices, particularly in high bandwidth usage situations, such as with wireless telephony.
Operational parameters other than transmit power can be optimized for better range performance under certain conditions. For example, antenna gain may be increased using a directional antenna, or more sophisticated receiver schemes can be employed. In some cases, limited range performance (or, equivalently, limited transmit power) may be accommodated by using repeaters to boost the signal power at an intermediate location between the source transmitter and destination receiver. However, each of these techniques may increase size, cost and circuit complexity, overall system complexity, and increase energy requirements.
Prior techniques involving Cooperative Team Communications (CTC) use the inherent multipath equalizer or combiner capability in the radio (e.g., at a Rake receiver). For CTC, the source radios coordinate transmissions such that the source transmitters concurrently transmit the same information-bearing signal on the same communication channel to the distant destination radios, so that the signals arrive at the destination radios are all within their multipath processing windows. As a consequence of the spatial diversity, the destination radios essentially treat the received signals as though they were different multipath signals. The multipath equalizer or combiner in each destination radio determines timing offsets among the received signals, and the received signals are time aligned by phase rotating the signals in accordance with the estimated timing offsets. The combined signal has a greater signal-to-noise ratio (SNR) than the individual received signals, permitting detection at a greater range or with a lower bit error rate.
The synchronous combining takes place in the Rake receiver, thus the CTC performance depends on the efficiency of the combining. The number of signals that may be integrated depends on the number of taps of the Rake receiver. Tap spacing needs to be pre-determined, designed, and implemented. Once designed and implemented, the Rake receiver is difficult, if not impossible, to adapt to different operating conditions.